The present invention relates to a photosensitive resin composition useful for the manufacture of relief plates for flexographic printing.
Hitherto, a rubber plate is chiefly used as a printing plate for flexographic printing. However, the prior art process for the manufacture of rubber plates for use in flexographic printing is extremely disadvantageous in the aspects of cost and working time since such process requires many working steps including those of exposure to light, development and etching for manufacturing a metallic original plate, manufacturing a mother plate from heat-resistant plastics, and manufacturing a rubber plate for use in flexographic printing by heating and pressing a rubbery material with the mother plate. In addition, the rubber plate obtained according to the prior art process is poor in accuracy and so requires such after-treatment as polishing on actual use. In the manufacture of rubber plates, a limitation naturally exists in the size of reproducible plates on account of the complexity of the apparatus and the steps so that rubber plates of a large size cannot be obtained.
On the other hand, there is proposed a process for the manufacture of relief printing plates wherein a photosensitive resin composition is utilized. For example, there is known a process for obtaining a relief plate which comprises shaping a mixture of a linear polymer, a monomer having at least one photopolymerizable olefinic double bond and a photopolymerization initiator into a sheet, exposing the sheet through an original to light, and washing off unexposed areas with a solvent. The use of a photosensitive resin composition comprised predominantly of an elastomeric polymer such as chloroprene rubber, nitrile rubber, polyurethane rubber, styrene-butadiene block copolymer or styrene-isoprene block copolymer; a photopolymerizable monomer and a photopolymerization inhibitor is also known (Japanese Patent provisional Publn. No. 37521/72) for improving the properties of relief plates.
A relief plate obtained according to such known conventional processes brings about a significantly good result as compared with a rubber plate. In the case of flexographic printing on materials with coarse surfaces or materials having uneven thickness, however, such relief plate is poor in contact with the materials to be printed and still fails to obtain satisfactory results.
As a result of various researches made to overcome the drawbacks of the prior art photosensitive resin compositions for flexographic printing plates, it has now been found that such drawbacks can be overcome by the use of a specific styrene-butadiene block copolymer and a specific prepolymer. The present invention has been accomplished on the basis of the above finding.